1. Field of the Invention
The present invention relates to a power supply device and an image forming apparatus using the same.
2. Description of the Related Art
Hitherto, an electrophotographic image forming apparatus is provided with a high-voltage bias generating circuit to generate a high-voltage bias, which is used in each step of an electrophotographic process. The high-voltage bias generating circuit outputs a positive DC bias, a negative DC bias, or an AC bias. For example, in a transfer device, a circuit capable of outputting positive and negative DC biases from one output terminal has been widely used (see Japanese Patent Laid-Open No. 2004-198481).
In order to decrease cost and improve miniaturization, parts of a plurality of high-voltage bias generating circuits may be unified. The inventors of the present invention have also invented a circuit configuration in which a control circuit to control voltage supplied to transformers constituting high-voltage bias generating circuits is shared thereby.
As an example, a transfer device of an image forming apparatus is described. Hitherto, a contact transfer device, which transfers a toner image formed on an image carrier onto a recording medium with a relatively low voltage by using a transfer member (e.g., a transfer roller) contacting the image carrier, has been developed. The contact transfer device typically includes a conductive transfer roller that contacts a rear side of a recording medium, and transfers a toner image on a photoconductive drum as an image carrier onto a recording medium by applying a relatively low bias voltage to the transfer roller. Such a contact transfer device has advantages that a battery can be miniaturized and that the amount of generated ozone is relatively small, because an applied voltage is lower than that in a noncontact transfer device using corona discharge.
The inventors of the present invention have also invented a high-voltage output circuit to output transfer biases of positive and negative polarities, in which output terminals of two DC voltage generating circuits generating positive and negative voltages are connected in series. In this high-voltage output circuit, if a control circuit to control voltage supplied to two transformers constituting the high-voltage output circuit is shared by the two transformers, overshoot, in which an output value temporarily exceeds a control value, may occur when switching between a positive bias and a negative bias is performed.
The overshoot partly changes the potential of the photoconductive drum. This may cause a false image. Further, if an output level is controlled by using a current value detected by a current detecting circuit, an abnormal current value may be detected due to the above-mentioned overshoot, so that control of the output level may become impossible.
The overshoot is believed to occur due to residual charge that has not been discharged from a capacitor for stabilizing voltage when an output voltage of an output circuit to output a DC voltage to the primary sides of the transformers of two DC voltage generating circuits is switched.
FIG. 5 is a timing chart showing a state where overshoot occurs when an output voltage is switched. For example, when output is switched from a negative bias to a positive bias, an input voltage to output a required positive bias level may be lower than an input voltage to output a negative bias, due to a difference in input/output characteristics of a positive transformer and a negative transformer. A control signal CNT supplied to an output circuit to output a DC voltage to the primary side of a transformer is controlled from a negative-bias level to a positive-bias level via an output stop state. Clock signals CLK1 and CLK2 drive switching elements on the primary sides of the negative and positive transformers, respectively. However, if the capacity of a voltage stabilizing capacitor is large, charge of a negative-bias level that was previously controlled remains in the capacitor, and outputting a positive bias starts before the voltage input to the primary side of the transformer is sufficiently decreased (when the voltage is higher than an input voltage of a required positive bias). Accordingly, an output transfer bias is brought into an overshoot state when a positive bias starts to be output, and rises to a level over a required control voltage.
As countermeasures against this problem, there is suggested a method in which a time period from when output of one of biases stops to when output of the other bias starts is made longer so that overshoot does not occur. In this method, however, as the time until the later bias output is started to be output at a required control voltage becomes longer, the time of a print sequence also becomes longer.
In the above-described example, a transfer device in which an output terminal is shared has been described. However, it is obvious that overshoot also occurs in another high-voltage bias using different output terminals on the basis of the same principle, if an output circuit to output a DC voltage to the primary sides of a plurality of transformers is shared.